1. Field of the Invention
The present invention is related to an apparatus and method for inspecting the picture elements of an active matrix type display, and more particularly to an apparatus and method for inspecting the picture elements of an active matrix type display, which is capable of enhancing picture element inspection accuracy upon picture element inspection of a liquid crystal display array (LCD array) or organic electro luminescent display array (EL array) having active matrix structures by canceling source switching element irregularities, measurement noise attributed to device drive signals, and irregularities in various elements in the measurement system.
2. Description of the Related Art
A conventional LCD array device and EL array device were assembled into respective modules, and a 100% inspection was performed by human visual observation.
In this inspection, a problem is that if the product is not assembled into its final form, an image cannot be displayed, and consequently, large expenses are incurred for nothing when a defect occurs, and, in addition, another problem is that inspection results are not reliable due to the fact that, because they are subjective inspections done by the naked eye, the evaluation criteria used by various inspectors are apt to lack uniformity, and inspection accuracy can be off the mark as a result of human fatigue.
Further, in an electrical automatic inspection apparatus and method, a certain electrical charge is applied to each picture element of an LCD array device and an EL array device, which are the devices targeted for inspection, and each picture element is inspected for malfunctions, broken wires, shorts and other such defects by reading this charge outside the device and evaluating the absolute value of the quantity of this charge.
However, the problem is that, in LCD display devices or EL display devices currently being developed, due to the high-temperature polysilicon processes and the low-temperature polysilicon processes which have come to be used in recent years, the irregularities in the characteristics of various elements inside a device resulting from manufacturing process-related problems are great.
A general explanation of a conventional liquid crystal display device, which constitutes the device to be inspected, will be given based on FIG. 6.
FIG. 6 is a diagram of an equivalent circuit of a polysilicon liquid crystal display 1 (active matrix type display), and polysilicon liquid crystal display 1 has a display device portion 3, in which a plurality of picture elements 2 are aligned in a matrix in the X-Y directions by LCD elements, and a horizontal driving circuit 4 and a vertical driving circuit 5 of the display device portion 3.
Each picture element 2 has an LCD element 6 and a switching element 7 (TFT: thin-film transistor), the respective sources of switching elements 7 are connected to horizontal driving circuit 4 via a plurality of source lines 8 (column select lines), and, in addition, the respective gates of switching elements 7 are connected to vertical driving circuit 5 via a plurality of gate lines 9 (row select lines). A picture element 2 is arranged at each intersecting portion 10 of the source lines 8 and gate lines 9.
Furthermore, polysilicon liquid crystal display 1 can be either a device in a state prior to encapsulating a liquid crystal in the LCD elements 6 thereof (that is, an active matrix type display board), or a device of a state subsequent to encapsulating a liquid crystal in the LCD elements 6 thereof (that is, an active matrix type display), and both of these can be treated as devices to be inspected.
Display device portion 3 can be treated as a device to be inspected independently, and, in addition, display device portion 3 can also be treated as a device to be inspected in a state in which it has been combined with at the least one side of either horizontal driving circuit 4 or vertical driving circuit 5.
Horizontal driving circuit 4 has a horizontal shift register 11, a video signal supply terminal 12, and a number of source switches 13 (column select switches, FET: field effect transistor) that corresponds to the number of columns of the display device portion 3 (9, from A1-A9, in the example shown in the figure).
Horizontal shift register 11 has a horizontal start signal (X-ST) supply terminal 14, a horizontal clock signal (X-CLK) supply terminal 15, and a number of horizontal flip-flop circuits 16 that corresponds to the number of columns of the display device portion 3 (3 in the example shown in the figure).
Video signal supply terminal 12 has an R video signal (VIDEO-R) supply terminal 17, a G video signal (VIDEO-G) supply terminal 18, and a B video signal (VIDEO-B) supply terminal 19.
Source switches 13 are connected between source lines 8 and horizontal shift register 11, and video signal supply terminal 12, and each column in display device portion 3 is selected by switching a source signal to a picture element 2.
Vertical driving circuit 5 has a vertical shift register 20, and vertical shift register 20 has a vertical start signal (Y-ST) supply terminal 21, a vertical clock signal (Y-CLK) supply terminal 22, and a number of vertical flip-flop circuits 23 that corresponds to the number of rows of the display device portion 3 (4 in the example shown in the figure).
In a conventional electrical automatic inspection device (not shown in the figure), which has a polysilicon liquid crystal display 1 of a configuration such as this as the device to be inspected, inspection is performed by applying a certain charge to each picture element 2, reading this charge in the exterior of the polysilicon liquid crystal display 1, and evaluating the absolute value of the quantity of this charge.
However, the problem is that, in polysilicon liquid crystal display 1 according to LCD display devices manufactured by either a high-temperature polysilicon process or a low-temperature polysilicon process, there are large characteristic irregularities in the various elements internal thereto due to problems related to the manufacturing process.
In particular, irregularities in the elements of source switches 13 (A1-A9) cannot be ignored, and the vertical stripes of when sampling a waveform outputted to an inspection device, which are the cause of these relatively large irregularities in source switches 13, are a big problem, and in an inspection method, wherein the absolute quantity of a read-out charge is evaluated simply, since there are numerous cases in which the side of the noise level resulting from these irregularities is larger than the level of the inspection signal, the results pose problems from the standpoint of inspection accuracy.
These source switch 13 irregularities are generated primarily by the nonuniformity of capacitance between a gate and a source brought on by the nonuniformity of ON resistance of each FET and the nonuniformity of the gate insulating film of each FET, and the nonuniformity of total impedance to picture element capacitance (LCD device 6) caused by interconnect resistance brought on by irregularities of delays between horizontal flip-flop circuits 16, which control the respective source switches 13, and the respective gate terminals, and different distances to the respective source switches 13 from terminals for flexible cable connections (not shown in the figure), resulting from the fact that a laser beam could not be uniformly irradiated over the entire device in the laser annealing process for growing a small amorphous silicon crystal up to a state, wherein it can be called a polysilicon.
Moreover, the problem is that, in line with making LCD and EL devices larger in recent years, each of these nonuniform items is moving in a direction in which the degree of this nonuniformity is increasing.
Further, when discharging the charge that is built up inside each picture element 2 inside a device (polysilicon liquid crystal display 1) being inspected, and sampling the discharge waveform thereof, a gate drive waveform, which leaks out through gate-source capacitance from the gate of each source switch 13 inside polysilicon liquid crystal display 1, and the crosstalk components of horizontal clock signals for driving horizontal shift register 11 are superimposed at the same timing as a picture element signal, and when the rising/falling edge of a vertical clock signal coincides with an image period, this also generates crosstalk, constituting factors that drastically lower the accuracy of picture element inspection.
A problem is that this crosstalk problem is becoming more apt to occur in line with making the polysilicon liquid crystal display 1 larger and higher in density.
Furthermore, when the noise originating in device drive signals from horizontal driving circuit 4 and vertical driving circuit 5, as well as the level of irregularities in each element of a test head or measurement device (not shown in the figure) is larger than a picture element inspection signal, there is the problem that the picture element inspection signal gets buried between such noise or irregularities levels, making detection impossible.
Furthermore, on the subject of picture element inspection devices for various displays, there are Japanese Patent Laid-open No. 5-313132, Japanese Patent Laid-open No. 6-43490, Japanese Patent Laid-open No. 6-59283, Japanese Patent Laid-open No. 7-287247, Japanese Patent Application Laid-open No. 10-96754, and Japanese Patent Application Laid-open No. 10-214065.